Chemical analysis, with photoelectron

The chemical composition of the surface can be evaluated from the integrated intensities of the core-level emissions [31]. Unfortunately, the accuracy of quantitative analysis with photoelectron spectroscopy is generally limited to a few percent even when good standards are available [31]. Therefore, even from a large number of measured samples it was not possible to observe a... 

Jonsson et al. used synchrotron radiation to study the kinetic energy of photoelectrons from the S(2p) core level in PEDOTPSS films in electron spectroscopy for chemical analysis (ESCA). Photoelectrons from the S(2p) core level can be used as a quantitative measure of sulfur atoms in the aromatic thiophene ring of PEDOT and sulfur atoms in the sulfonic acid unit of PSS in the top layer of the film. By variation of the energy of the photons, the escape depth of photoelectrons from the polymer film can be tuned and a depth profile of the film can be obtained. Using this technique the authors demonstrated that pristine PEDOTPSS films have a PSS rich surface. With a photon energy of 270 eV, 95% of the signal stems from the outmost 15 A and a clear dominance of PSS is observed (see Table 9.4). When NMP and sorbitol are used as additives and the film is dried at room temperature, no PSS rich... 

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

We shall concern ourselves here with the use of an X-ray probe as a surface analysis technique in X-ray photoelectron spectroscopy (XPS) also known as Electron Spectroscopy for Chemical Analysis (ESCA). High energy photons constitute the XPS probe, which are less damaging than an electron probe, therefore XPS is the favoured technique for the analysis of the surface chemistry of radiation sensitive materials. The X-ray probe has the disadvantage that, unlike an electron beam, it cannot be focussed to permit high spatial resolution imaging of the surface. 

XPS or ESCA (electron spectroscopy for chemical analysis) is a surface sensitive technique that only probes the outer atomic layers of a sample. It is very useful tool to study polymer surfaces [91]. An XPS spectrum is created by focusing a monochromatic beam of soft (low-energy) X-rays onto a surface. The X-rays cause electrons (photoelectrons) with characteristic energies to be ejected from an electronic core level. XPS, which may have a lateral resolution of ca. 1-10 pm, probes about the top 50 A of a surface. 

The work of Siegbahn s group who, in the 1950s, improved the energy resolution of electron spectrometers and combined it with X-ray sources. This led to a technique called electron spectroscopy for chemical analysis (ESCA), nowadays more commonly referred to as X-ray photoelectron spectroscopy (XPS) [6]. Siegbahn received the Nobel Prize for his work in 1981. Commercial instruments have been available since the early seventies. 

This technique is also known as electron spectroscopy for chemical analysis (ESCA). Although it is concerned with the detection of electrons, it is discussed here because the way in which the photoelectrons are produced is fundamental to the XRF process. As described above, an incident X-ray photon produces an excited ion by ejecting an inner shell electron. The excited... 

Although not capable of the micrometer-sized lateral resolutions available with the aforementioned techniques, the surface spectroscopy, electron spectroscopy for chemical analysis (ESCA), also deserves mention. The ESCA experiment involves the use of X-rays rather than electrons to eject core electrons (photoelectrons), and it has comparable surface specificity and sensitivity to that of Auger electron spectroscopy (AES) (25, 26, 29). The principal advantage of ESCA relative to AES is that small... 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

The XPS mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft Mg or Al anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize die core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their BE. The resulting XP spectrum is a characteristic set of peaks for a specific element, with BE as the abscissa and counts per unit time as... 

In this review the term X-ray photoelectron spectroscopy (XPS) will be used rather than the term adapted by Sieghahn, namely electron spectroscopy for chemical analysis (ESCA), to describe the surface sensitive electron spectroscopy under discussion. The fact that this symposium is taking place on the topic of industrial applications of surface analysis reflects the growth of the application of different areas of surface sciences in practical industrial and technological materials science areas. Initially, scientists working in XPS were concerned with establishing the fundamentals of the physics associated with the processes involved in the... 

X-ray photoelectron spectroscopy (XPS), with a defunct "propaganda" name of electron spectroscopy for chemical analysis (ESCA), was developed by Siegbahn186 in 1954 it measures the elemental composition and valence state of elements in solids (atomic number Z = 3 to Z = 92) to within about 5 to 10 nm of their surface by impinging X-rays, typically monochromatized A1 Ka (Ex = 1.4867 keV and lx = 0.83386 nm) in a beam of 0.02- to 0.2-mm diameter, onto a sample surface in ultra-high vacuum and measures to within 0.25 eV... 

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